Ambient and low temperature curable coating compositions are desirable in many applications. For example, such coating compositions are, in at least some cases, preferable over, for example, thermally-cured or radiation cured coating compositions because (i) little or no energy is required to cure the composition, (ii) the materials from which some substrates are constructed cannot withstand elevated temperature cure conditions, and/or (iii) large or complex articles, such as automobiles and the like, to be coated may not be convenient for processing through thermal or radiation cure equipment.
Isocyanate-based ambient and low temperature curable coating compositions are common. Isocyanates reacted with amine-functional compounds yield ureas ((NH2)2CO) and isocyanates reacted with a hydroxyl functional compound yield urethanes (NHCOO). Polyurethane resins are suited for, and used in, a variety of commercial coatings, such as automotive refinish coatings, including primer sealers, pigmented topcoats and clearcoats. Two component coating compositions that, when combined, react to form a polyurethane are often curable at ambient or low temperatures and are typically catalyzed with tertiary amines or an organotin catalyst, such as dibutyl tin dilaurate. A major concern with polyurethane based coatings, however, is human exposure and proper safety when handling isocyanate-based materials, particularly low molecular weight isocyanates.
As a result, various isocyanate-free ambient and low temperature cure technologies have been explored as alternatives. For example, acrylic polyanhydrides have been used as crosslinkers for hydroxyl functional compounds and amine functional compounds. Unfortunately, these crosslinkers have been of relatively high viscosity as compared to isocyanates. The higher viscosity has necessitated the use of higher amounts of volatile organic compounds (VOCs) to achieve composition viscosities convenient for application by spraying, for example. As will be appreciated, however, use of larger amounts of VOCs is undesirable from, for example, an environmental standpoint. In addition, some of the performance properties of such coatings may be inferior to isocyanate-based compositions.
As a result, it would be desirable to provide substantially isocyanate-free ambient and low temperature cure coating compositions that can sprayable while being low in VOC content and which can provide coatings having good properties, such as good hardness and solvent resistance.